A wireless communication system generally refers to a communication system which makes a wireless data transmission connection possible between a wireless communication device (MS, mobile station) and stationary parts of the system, the user of the wireless communication device moving within the operating range of the system. A typical system is a public land mobile network PLMN. A majority of wireless communication systems belongs to so-called second-generation mobile communication systems, of which an example to be mentioned is the widely known circuit switched GSM mobile communication system (Global System for Mobile Telecommunications). The present invention is particularly suitable for mobile communication systems under development. As an example of such a communication system, in this description, the GPRS system (General Packet Radio Service) will be used, which is presently under development. It is obvious that the invention can also be applied in other systems which are based on packet systems, such as the GPRS system, or which utilize it (UMTS, Universal Mobile Telecommunication System).
In modern general mobile communication networks based on a cellular system, the system consists, in a known way, of several mobile stations (MS) applying the system, such as mobile phones, and a stationary base station subsystem (BSS). This base station subsystem normally consists of several base transceiver stations (BTS) distributed over a geographical area, and each base station serves a cell which consists of at least a part of this geographical area.
For example, in the GSM system, communication between communication devices, such as a mobile station and a base station, is carried on logical radio channels. A packet switched system based on the GSM system, the GSM GPRS system, makes communication more efficient, since the same logical radio channel can be used by several different mobile subscribers. Data is transmitted only when necessary, and the logical radio channel is not allocated to communication between one mobile station and one base station only. In the system, there is a so-called virtual data transmission connection between the mobile station and the GPRS system. The operating environment of the system is known as such and is widely defined in the ETSI standards, wherein a more detailed description will not be necessary. For using GPRS services, the MS will first log in the network (GPRS attach). The login forms a logical link between the wireless communication device and the serving GPRS support node SGSN.
The trouble-proof operation of the communication network and the efficient utilization of the available resources are only possible, if the power levels used in the transmissions by e.g. the base stations are as optimal as possible. In addition to this, demands are continuously set for the power consumption of the mobile station itself.
The basic idea of the GPRS system is to apply packet switched resource allocation, wherein resources, e.g. a logical radio channel, are allocated when there is a need to transmit and receive data and information. Thus, the use of available resources can be optimized to be as efficient as possible when compared with e.g. circuit switched GSM technology. The GPRS is designed to support applications which utilize discontinuous data transmission containing, intermittently, even large quantities of data. In the GPRS system, the allocation of channels is flexible, and 1 to 8 time slots of a channel, i.e. 1 to 8 physical channels, can be allocated to each wireless communication device within the scope of a TDMA frame. The term TDMA (Time Division Multiple Access) refers to the division of a radio frequency channel into successive time slots on the time level, known as such. The same resources can be allocated to several active mobile stations. U communication (uplink, i.e. communication from the mobile station to the base station) and D communication (downlink, i.e. communication from the base station to the mobile station) can be separately allocated to different users. In each time slot, an information packet with a definite duration is transmitted as a radio frequency burst consisting of a set of modulated bits. The time slots are primarily used as control channels (CCH) and traffic channels (TCH). The traffic channels are primarily used for the transmission of speech and data, and the control channels are used for signalling between the BTS and the MS. One logical control channel is the BCCH (Broadcast Control Channel), which is used for broadcasting detailed information related to the communication network or the cell. This information relates, for example, to the frequencies used by the cell in question and by the adjacent cell, as well as to the surrounding cells, frequency hopping, channel combinations, and paging groups.
The most significant difference between the GPRS system and the circuit switched GSM system is the packet-based communication. A physical channel, so-called packet data channel PDCH, is allocated to packet communication in the GPRS system based on a cellular system. The logical channels contained in the PDCH channel (e.g. PDTCH/D, Downlink Packet Data Traffic Channel) are collected into a frame structure (Multiframe) consisting of repeatedly transmitted 52 TDMA frames (20) which are divided (PCDH/F, Full rate PDCH channel) further into 12 successive blocks (radio blocks), each comprising four frames (TDMA FRAME) as well as four extra frames (IDLE FRAME). The blocks 10 are indicated by blocks B0 to B11, respectively, as shown in FIG. 1. In FIG. 1, the idle frames are also indicated with the reference X. In D communication, these can be used for signalling.
The blocks 10 are further divided into the following parts shown in FIG. 2: MAC header (Medium Access Control Header), RLC data block (Radio Link Control Data Block) or RLC/MAC control block, and BCS block (Block Check Sequence). The RLC data block contains an RLC header and RLC data. The MAC header also comprises a USF field (Uplink State Flag). The terms LLC, RLC and MAC refer to different levels (protocol layers) in a protocol structure according to an OSI model (Open Structured Interface), known as such, which are described in more detail in the ETSI standard specifications.
For multiple access in D communication, a temporary flow identifier TFI is used in the data header. Each RLC header contains a TFI which is used to indicate the blocks addressed to a specific, given mobile station MS. According to the GPRS system, all the communication devices MS, which are waiting for data transmitted to them on the channel jointly allocated to them, receive all the blocks, including the RLC blocks, interpret the received information and the TFI, and select the blocks addressed to them.
During channel allocation, at the beginning of the data transmission, the MS is informed of the allocated blocks, during which transmission and reception is possible. The connection is set up either by means of a packet channel request PCR, which is transmitted by the mobile station to request for resources from the base transceiver station on the packet random access channel PRACH, or by means of a packet paging request PPR transmitted by the base transceiver station. The resources allocated for data transmission are indicated, for example, by means of a packet uplink assignment PUA or a packet downlink assignment PDA, containing e.g. a list of the available PDCH channels, the value of the USF field to be used, and a determined individual temporary flow indicator TFI which is added to each RLC block used in the communication. In the GPRS system, the mobile stations must be continuously ready for packet communication (Temporary Block Flow, TBF), wherein they must quickly shift from the so-called idle mode to the so-called packet transfer mode.
In digital TDMA systems, such as the GSM system, the mobile communication device continuously measures the signal strength on radio channels of the serving base station and the adjacent base stations, and transmits a measuring report to the base station. The measurements taken by the mobile communication device are typically, and in a way known as such, related to power control used in D communication, cell reselection or handover. Power control refers, for example, to the transmission power level used by the BTS for transmitting a radio signal to the MS.
In a way known as such, the MS maintains information about the received signal strength of the used BCCH radio channel by means of an average. This information and the calculated C parameter (normalized Rx level) are utilized in decisions related to cell reselection. The signal strength is measured in units of dBm. The averaging is based on at least five, normally six samples taken from the radio channel during one multiframe structure (52 TDMA frames). The rules for power control, the measurement, the calculation of the C value, and the modes A and B for the power control, which will be described in the following, are also defined in more detail for example in the ETSI specification (European Telecommunications Standards Institute) 3GPP TS 05.08 V8.6.0 (2000-09), particularly in the chapter: 10.2. RF Power Control, which is incorporated herein as a reference. The present invention for calculating the ARR parameter utilizes these same samples SSn of the received signal level, Rx level.
One known technology used in mobile stations for controlling the gain of the receiver is the so-called AGC method (Automatic Gain Control), which has the function of tracing the effects caused by the mobility and the environment of the MS, for example, on the radio wave in connection with multipath propagation. These include, for example, reflections, frequency dependent drop-outs and attenuations, as well as various slow and fast changes. The signal levels are also changed as a result of BTS power control in D communication. In the invention, in turn, the operation of the AGC is based on tracing the Rx levels of the received signal, when the MS is receiving on the PDTCH channel. Both the PDTCH and the BCCH channels are traced. The MS must be capable of tracing changes in the received downlink signal in order to be able to interpret the information of the block, for example the TFI data, so that the MS can determine whether the block is addressed to it for reception. The aim is to set the gain level of the received analog RF signal before the AD (analog-to-digital) conversion and on a reference level suitable for input in the receiver of the MS. The dynamic range of the receiver is typically defined to be restricted above (15 dB) and below (20 dB) a specific reference level. The difference in the power levels between the frames can be as great as 30 dB.
According to the rules of the ETSI/3GPP specification in the GPRS system, constant power control is used for D communication on PDCH channels, which are used, for example, as PBCCH and PCCCH control channels. The power level can be lower on the PCCCH than on the BCCH channel (Broadcast Control Channel), the difference (Pb) being indicated on the PBCCH channel. The PTCCH/D channel (Packet Transfer Control Channel) applies the same power level as the PBCCH channel, or if this is missing, the same power level as the BCCH channel. Thus, different frequencies must be allocated for the PCCCH and for the BCCH, because the BCCH frequency always has a constant power level. In the blocks of the other PCDH channels, it is possible to use power control in D communication. The power level is always the same during the bursts (4 bursts) of a single radio block.
In a known manner, two different control modes are used for the power control of the PDCH blocks: Mode A, and Mode B when fixed allocation is used only. In Mode A, the variation in the output power of the BTS is limited, whereas in Mode B, the whole range of variation of the output power of the BTS is in use. The used mode is indicated to the MS in connection with resource allocation (so-called Assignment message). In both modes, the so-called P0 parameter is used, which indicates the power reduction compared to the BCCH channel whose carrier can be continuously traced. The parameter P0 is also indicated in the Assignment message. The used mode and the value of P0 are not changed without new assignment messages when the MS is in packet transfer mode.
Each block of the PDTCH/D channel contains a PR field in the MAC header, to indicate the power level of said block, if it is in use and power control is used. There are two PR modes. In PR mode A (appended Table 1), the PR value is calculated on the basis of the output power level used by the BTS, when the block is addressed to a specific receiving MS. A P0 value is determined for each MS.
The PR value is calculated in relation to the P0 value of the MS in question. In PR mode B (appended Table 2), the same power level is used for each block and for each mobile station on the same PDCH channel communicating (TBF) with the BTS. The P0 value indicates the power level of the first block on the PDCH, and the PR value is calculated in relation to the level of the BCCH.
TABLE 1BitsPower reduction, Mode A0 00 to 2 dB lower than BCCH level - P00 14 to 6 dB lower than BCCH level - P01 08 to 10 dB lower than BCCH level - P01 1Not usable
TABLE 2BitsPower reduction, PR Mode B0 0 0 to 6 dB lower than BCCH level0 1 8 to 14 dB lower than BCCH level1 016 to 22 dB lower than BCCH level1 124 to 30 dB lower than BCCH level
According to the GPRS specifications, the BTS transmits to the mobile stations in packet transfer mode at least one reference block, which conforms to the used mode, at certain intervals (78 TDMA frames). The reference block is also transmitted when the power control is not in use. In addition to this, if power control is used, in PR Mode A and PR Mode B, the reference block contains a usable PR field. In PR Mode A, valid, usable blocks only include blocks addressed to the MS in question and having a valid PR value, but in PR Mode B, the block does not need to be addressed to the MS in question.
For PDTCH configuration, i.e., in a situation in which the MS is in GPRS packet transfer mode, no procedure is defined in prior art, for determining the reference value for the AGC function. It is an aim of the present invention to present this method.